A computer network is a geographically distributed collection of interconnected communication links and segments for transporting data between nodes, such as computers. Many types of network segments are available, with the types ranging from local area networks (LAN) to wide area networks (WAN). For example, the LAN may typically connect personal computers and workstations over dedicated, private communications links, whereas the WAN may connect large numbers of nodes over long-distance communications links, such as common carrier telephone lines. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes on various networks. The nodes typically communicate over the network by exchanging discrete frames or packets of data according to predefined protocols. In this context, a protocol consists of a set of rules defining how the nodes interact with each other.
Computer networks may be further interconnected by an intermediate node or device, such as a switch or router, having a plurality of ports that may be coupled to the networks. To interconnect dispersed computer networks and/or provide Internet connectivity, many organizations rely on the infrastructure and facilities of Internet Service Providers (ISPs). ISPs typically own one or more backbone networks that are configured to provide high-speed connection to the Internet. To interconnect private networks that are geographically diverse, an organization may subscribe to one or more ISPs and couple each of its private networks to the ISP's equipment. Here, the router may be utilized to interconnect a plurality of private networks or subscribers to an IP “backbone” network. Routers typically operate at the network layer of a communications protocol stack, such as the internetwork layer of the Transmission Control Protocol/Internet Protocol (TCP/IP) communications architecture.
Simple networks may be constructed using general-purpose routers interconnected by links owned or leased by ISPs. As networks become more complex with greater numbers of elements, additional structure may be required. In a complex network, structure can be imposed on routers by assigning specific jobs to particular routers. A common approach for ISP networks is to divide assignments among access routers and backbone routers. An access router provides individual subscribers access to the network by way of large numbers of relatively low-speed ports connected to the subscribers. Backbone routers, on the other hand, provide transports to Internet backbones and are configured to provide high forwarding rates on fast interfaces. ISPs may impose further physical structure on their networks by organizing them into points of presence (POP). An ISP network usually consists of a number of POPs, each of which comprises a physical location wherein a set of access and backbone routers is located.
As Internet traffic increases, the demand for access routers to handle increased density and backbone routers to handle greater throughput becomes more important. In this context, increased density denotes a greater number of subscriber ports that can be terminated on a single router. Such requirements can be met most efficiently with platforms designed for specific applications. An example of such a specifically designed platform is an aggregation router. The aggregation router, or “aggregator”, is an access router configured to provide high quality of service (QoS) and guaranteed bandwidth for both data and voice traffic destined for the Internet. The aggregator also provides a high degree of security for such traffic. These functions are considered “high-touch” features that necessitate substantial processing of the traffic by the router.
More notably, the aggregator is configured to accommodate increased density by aggregating a large number of leased lines from ISP subscribers onto a few trunk lines coupled to an Internet backbone. Increased density has a number of advantages for an ISP, including conservation of floor space, simplified network management and improved statistical performance of the network. Real estate (i.e., floor space) in a POP is typically expensive and costs associated with floor space may be lowered by reducing the number of racks needed to terminate a large number of subscriber connections. Network management may be simplified by deploying a smaller number of larger routers. Moreover, larger numbers of interfaces on the access router improve the statistical performance of a network. Packet networks are usually designed to take advantage of statistical multiplexing, capitalizing on the fact that not all links are busy all of the time. The use of larger numbers of interfaces reduces the chances that a “fluke” burst of traffic from many sources at once will cause temporary network congestion.
In addition to deployment at a POP, the aggregator may be deployed in a telephone company central office. The large numbers of subscribers connected to input interface ports of the aggregator are typically small to medium sized businesses that conduct a substantial portion of their operations “on-line”, e.g., over the Internet. Each of these subscribers may connect to the aggregator over a high reliability link connection that is typically leased from, e.g., a telephone company provider. The subscriber traffic received at the input interfaces is funneled onto at least one trunk interface. That is, the aggregator essentially functions as a large “fan-in” device wherein a plurality (e.g., thousands) of relatively low-speed subscriber input links is aggregated onto a single, high-speed output trunk to a backbone network of the Internet.
Broadly stated, each input link may comprise a T1 or T3 connection, whereas the output trunk interface connection may comprise an OC-12 connection. A T1 connection has a data rate of 1.5 megabits per seconds (Mbps) and there are preferably 28 T1s to each T3 connection; a T3 connection thus has an effective data rate of approximately 42 Mbps. On the other hand, the effective data rate of an OC-12 trunk interface connection is typically 622 Mbps. As noted, the aggregator receives a plurality of subscriber inputs (e.g., 1000 T1 lines) and aggregates them onto a single output trunk (e.g., an OC-12 link). However, not all T1 links are constantly in use and, through the use of statistical multiplexing, the number of input subscriber lines that can be disposed over a single output trunk at one time can be determined. For example, although the effective data input rate of 1000 T1 lines is greater than 1 gigabit per second (Gbps), the subset of T1 lines that is constantly in use (e.g., approximately 400 T1 lines) may be supported over a single 622 Mbps link (OC-12).
Examples of intermediate network device platforms that may be used as aggregation routers include the 7200 Series and 12000 Series systems available from Cisco Systems, Inc. The Cisco 7200 system embodies a centralized network processing engine architecture that utilizes a shared bus as its switching fabric. The Cisco 12000 gigabit switch router (GSR) system, on the other hand, is based on a high-speed distributed routing architecture that utilizes a crossbar switch fabric having point-to-point links as its switching core. The present invention is directed, in part, to an aggregation router having a centralized routing architecture coupled to a point-to-point switch fabric for servicing a plurality of subscriber input links.
As noted, floor space is a precious commodity within the telecommunications industry and, in particular, within an ISP. As a result, the industry typically employs conventional “channelization” techniques to address this issue. That is, a “front end” subsystem may be used to multiplex (aggregate) a plurality of telephone calls (e.g., 24×28 DS0 links) over a single high-capacity (e.g., T3) output link by periodically sampling the calls and placing them over the output link. The T3 link (and other similar links) is then provided as an input to the aggregator. The architecture of the novel aggregation router described herein is adapted to aggregate a plurality of high-capacity, e.g., T3, links onto a higher-capacity, e.g., OC-12, output link. The present invention is further directed to an aggregation router architecture that occupies a relatively small footprint.